That really could be the difference -- the lack of decoupling capacitor across the rails close to the chip allows the voltage on the d6 databus/pins to spike nastily when the other ICs are better behaved. It does say "additional signals", meaning noise. Nice one

"It's not a bug, it's a design feature".

Really? Is it really that we needed to squeeze in a resistor down there, and there wasn't space to fold the capacitor over it?